1. Field of the Invention
The present invention relates to a system for transferring variable length cells under an asynchronous transfer mode (ATM), a switch apparatus handling variable length cells, and an exchange handling variable length cells.
In recent years, there have been much research and development conducted on ATM technology in view of its suitability for large volume, high speed data transmission and exchange. Fixed length cells of 53 bytes have been proposed as the standard unit for transfer in ATM systems, but when such fixed length cells are used, it is not possible to make efficient use of the transmission band due to the high proportion of the control data. Accordingly, improvement has been desired.
2. Description of the Related Art
In the conventional mode of ATM transfer, the exchange performs high speed switching under control by hardware, so fixed length cells are used as the units of transfer. By using fixed length cells as the unit of transfer, the following advantages are obtained in an ATM system: That is, since hardware is used for controlling the transfer, despite the fact that the ATM is a label multiplex mode, it is possible to transfer more than the so-called H4 channel (about 135 Mbit/sec) of user information at the user-network interface (UNI). Further, due to the label multiplex mode, the user information can be transferred at any speed.
As explained later with reference to the drawings, however, in the fixed length ATM transfer mode, a header and trailer are coded in each layer and accommodated in the ATM cell payload. In this case, since a 5 octet header field is allocated in each fixed length ATM cell, a large header field is required for transferring user information and therefore the efficiency of transmission is poor.
Further, since the transfer capacity of each cell is 44 octets of information, even if the data to be transferred is less than 44 octets, it is necessary to create an empty region (ineffective octets) in the cell to bring the length to 53 octets for the transfer. This is uneconomical.
Taking as an example the case of generation of 88 octets (oct) of user information at a higher layer, the information becomes the convergence sublayer (CS)--protocol data unit (PDU) payload as it is. A 4-octet CS-PDU header and CS-PDU trailer are added before and after the same to bring the total length to 96 octets. When this is transferred to the segmentation and reassembly (SAR) sublayer, it is successively disassembled into 44 octet SAR-PDU payloads. In this example, 44 octets of effective information of the CS layer are loaded in the first two SAR-PDU payloads. The third SAR-PDU payload, however, is loaded with only 8 octets of effective information. The remaining 36 octets (44-8=36) of space is loaded with ineffective information. At the ATM layer, these three SAR-PDU's are used to make three ATM cells which are then transmitted over the transmission line. In this way, when transferring user information, there is the problem that the finally transferred amount of data becomes considerably larger than the amount of the original user information.
Further, when transferring voice information using fixed length cells as in the past, it is demanded to transfer the information once every 125 .mu.sec in periods when voices are being transmitted due to the nature of voice information. In this case, when transferring 53 octet (byte) fixed length cells every 125 .mu.sec, there is the problem that transmission is not possible with a low bit rate channel (for example, 1.5 Mbps channel). A channel rate of at least 3.4 Mbps is required.